A color cathode-ray tube is composed of a glass envelope comprising an approximately rectangular front face joined to a funnel-shaped rear part terminating in a cylindrical neck. An electron gun is located in the neck and generates electron beams intended to form a color image on a luminescent screen deposited on the internal surface of the front panel. The electron beams are brought into correspondence with the phosphors on the luminescent screen by means of a perforated metal mask, called a color selection mask. The mask is attached to an approximately rectangular rigid frame with two pairs of opposed sides, one pair of short sides and one pair of long sides. An internal magnetic screen is generally placed in the funnel-shaped part of the envelope and is joined to the rear of the frame. The primary objective of this magnetic screen is to reduce the influence of the components of the Earth's magnetic field on the paths of the electron beams so that the angles of incidence of the beams on the selection mask are not significantly modified by these components; otherwise, the points of impact of the beams on the screen will be shifted and illuminate phosphors of a color other than that desired and cause a defect known as a registration error.
Cathode-ray tubes (CRTs) are all the more sensitive to these problems of registration errors with increasing angle of deflection of the electron beams. With the current trend in CRT industry to reduce the depth of the tube in relation to the size of the screen, the propensity for external magnetic fields to cause registration errors in the screen peripheral areas is greater because the required electron beam deflection angle becomes greater in such tubes.
Additionally, the CRT market is seeing a greater interest in tubes having flat faceplate panel. Likewise product with such a geometry will also show an enhanced propensity for external magnetic fields to cause registration errors in the screen peripheral because the electron beam designated for the peripheral area will have to travel a greater distance to such screen areas than in comparable spherical tubes. To correct geometrical distortions of the screen images, one may find it necessary to utilize magnets which are strategically placed outside the tube.
In short, because the angle of deflection of the electron beams of these trendier tubes is greater than in comparable spherical panel tubes with the normal depth, the path travelled by the electron beams emanating from the gun is comparatively longer. This makes them more sensitive to the influence of the Earth's magnetic field.
Three directions of the Earth's field must be taken into consideration. The axial (north/south) field is manifested parallel to the longitudinal axis of the tube, the lateral (east/west) field along the horizontal axis and the vertical field along the vertical axis.
The vertical field does not pose a major problem for registration as it is virtually constant within broad geographical areas. On the other hand, it is necessary to screen the inside of the tube against the axial and lateral fields since their influence depends on the location of the tube and, on the position in which it lies.
The magnetic screens or internal magnetic shields of the prior art are designed to have a shape matching as closely as possible the internal surface of the funnel-shaped part of the envelope. Further, the apertures, their shapes and their numbers are adapted to compensate and reduce the influences of the Earth's field on electron beam misregister. In its front part, the internal magnetic screen is closed by the color selection mask. In its rear part it has an aperture so as to allow the electron beams to pass through.
It has been found that tubes of reduced depth, that is to say tubes for which the angle of horizontal deflection of the electron beams is greater than 108°, have a very high sensitivity to the Earth's magnetic field and that it is necessary for the magnetic screen to extend towards the rear of the tube as close as possible to the deflection device. However, it is no longer possible in this case to place magnets for correcting the geometry of the image in front of the deflection device because the field lines of the magnets will need to be position in the vicinity of the internal magnetic screen and with such placement the magnetic will not have an influence on the electron beams.
Therefore, there is a need for a color cathode-ray tube having improved magnetic screening capabilities.